Modern coffee machines in some cases include a coffee grinder, which is fed with coffee beans delivered from a fixed or removable coffee bean container. When a brewing cycle is initiated, a certain amount of coffee beans is ground to produce coffee powder. The coffee powder is loaded in a brewing chamber of a brewing unit, compacted and extracted by means of hot and pressurized water to produce a coffee-based beverage.
Some known grinding devices comprise a volumetric dosing system: the amount of coffee ground per brewing cycle is determined based on the volume of ground powder. WO 2011/070502 discloses a grinder with a dosing chamber wherein ground coffee powder is accumulated. Grinding is stopped when a sufficient amount of ground powder is accumulated in the dosing chamber. The coffee powder is then loaded from the dosing chamber into a brewing unit.
In other machines, the amount of coffee beans to be ground at each grinding cycle is determined using a dosing chamber arranged upstream of the grinder, so that a fixed amount of coffee beans is delivered to the grinder at each grinding cycle. WO 2011/102715 discloses a coffee-producing machine having a dosing chamber which is filled with coffee beans from a removable coffee container and wherefrom the coffee beans are delivered to the coffee grinder.
In some coffee machines the grinding process is performed by keeping the rotary grinding wheels or disks into rotation for a fixed time or a fixed number of revolutions, usually measured by means of a sensor arrangement, for example using one or more magnets rotating with a rotary grinding wheel or grinding disk and one or more Hall-effect sensors.
In some known machines the grinding cycle includes an active grinding period, during which the coffee beans are actually ground and reduced into powder form, and an idle rotation period, when the grinder is allowed to rotate freely in order to remove the residual coffee powder from the grinding disks, while no coffee beans are delivered from the chamber, so that if the kind of coffee used is changed from one grinding cycle to the next (e.g. shifting from regular coffee to decaffeinated coffee or vice-versa), coffee of the first type does not contaminate coffee of the second type.
The way of controlling this kind of grinding process is not satisfactory, because depending upon the actual duration of the active grinding period, an insufficient or a too long idle rotating period can sometimes be performed. If the grinding cycle is too short, the risk of contamination is not eliminated. A cycle which is too long is unsuitable as the user has to unnecessarily wait for a process to be completed, which in actual fact is useless. The efficiency of the grinding disks is hardly predictable, due both to wear, which negatively affects the efficiency of the grinding disks, as well as to the unpredictability of the process of manufacturing the grinding disks. In some situations, the efficiency of the grinding disk can be less than expected, which will result in contamination. In some cases, e.g. if at the end of a grinding cycle of pre-determined duration the grinder has still not completed the grinding of the required quantity of material due to an unexpectedly low grinding efficiency, the grinding cycle can be aborted, as the control unit wrongly interprets this situation as a lack of material in the grinder.
Accordingly there is a need for improving the method of operating a grinder, in particular a coffee grinder. Specifically, a need exists of improving the way of controlling a grinding cycle comprised of a first active grinding period and a second idle rotating period for empting the grinder before starting a subsequent grinding cycle.